1. Field of the Invention
The present invention relates to an anti-skid control system for controlling the braking force applied to each driven wheel in a braking operation of an automotive vehicle having four driven wheels to prevent each driven wheel from being locked, and more particularly to an anti-skid control system for estimating a coefficient of friction of a road surface, and controlling the braking force in accordance with the estimated coefficient of friction.
2. Description of the Prior Art
In general, an ordinary passenger vehicle has a pair of road wheels at each of its front and rear sides. Either the front road wheels or the rear road wheels of that vehicle are operatively connected with the internal combustion engine to be driven directly thereby, while the rest of the wheels are not connected with the engine so as to be served as non-driven wheels. A vehicle having the driven wheels at its front side is called a front drive vehicle, while a vehicle having the driven wheels at its rear side is called a rear drive vehicle. Whereas, a vehicle having the driven wheels at both of the front and rear sides is called a four-wheel drive (4WD) vehicle. Further, there has been provided a differential gear mechanism for compensating a difference between the rotational speeds of the right and left driven wheels so as to ensure a smooth driving of the vehicle. In other words, the right and left driven wheels are controlled by means of the differential gear mechanism such that an equal torque is transmitted to the right and left driven wheels, respectively. As for a driving system of the four-wheel drive vehicle, there are known various types of the system, such as a part time system, full time system, of the like. According to the full time system, the front driven wheels and the rear driven wheels are connected by a differential gear mechanism, i.e., so called center differential gear. Also, in order to prevent a trouble which may be caused when only either one of the driven wheels slips, a limited slip differential mechanism (LSD) is employed for limiting the differential operation and increasing the driving force, with a mechanism for generating a frictional torque, or a mechanical clutch mechanism, provided in the differential gear mechanism.
The coefficient of friction between the road surface and the road wheels is varied in dependence upon the kinds of the road wheels, the road surface condition and the like. Especially, the coefficient of friction (herein, abbreviated as CF) is varied to a large extent in dependence upon the conditions of the road on which the vehicle is running, such as a dry road surface and a wet road surface. Thus, it is very important to detect the coefficient of friction of the road surface (hereinafter, referred to as the road-CF). In this respect, when the vehicle is running, it is impossible to directly detect the road-CF. Therefore, according to an apparatus disclosed in Japanese Patent Laid-open publication No.60-35647 for example, the road-CF is determined by comparing in magnitude a wheel speed and a wheel acceleration, with a standard speed and a standard acceleration of each wheel, respectively. However, in the apparatus disclosed in that publication, the road-CF is supposed to be estimated after the pressure decreasing operation in a wheel cylinder of each road wheel has been initiated, so that the road-CF is estimated only when the hydraulic braking pressure is being controlled. Therefore, it is impossible to estimate the road-CF before the pressure decreasing operation starts.
According to a hydraulic braking system employing a proportional control electromagnetic valve as disclosed in Japanese Patent Laid-open publication No.3-208758, it has been proposed to detect the road-CF at the time of starting a braking pressure control for providing an appropriate braking pressure control at the time of starting the same in accordance with the road conditions. In practice, it is so arranged that the road-CF is estimated on the basis of an acceleration of a standard speed which is provided at the time when means for determining the necessity of the braking pressure control has determined the same to be initiated. That is, an estimated vehicle speed Vs is calculated on the basis of the maximum speed of the wheel speeds of the four road wheels before the braking pressure control is initiated, and an acceleration Vsd of the estimated vehicle speed Vs is calculated. Then, the road-CF is estimated on the basis of the acceleration Vsd thereby to provide a hydraulic braking pressure applied to each wheel cylinder.
In the above-described publication, however, the value of the hydraulic braking pressure which is provided when the braking pressure control is initiated, is unknown, and the road-CF which is estimated on the basis of a variation of the estimated vehicle speed Vs per a certain period of time, can not be estimated, if all the road wheels are locked simultaneously. In order to solve this problem, an anti-skid control apparatus has been proposed in Japanese Patent Laid-open publication No.5-131912. This apparatus includes means for calculating the variation of the estimated vehicle speed per a certain period of time, means for estimating the coefficient of friction of the road by comparing the variation with a predetermined value, braking force control means for controlling an actuator in accordance with at least an estimated result of the coefficient of friction estimation means to control the hydraulic braking pressure fed to a wheel cylinder thereby to control the braking force applied to a road wheel, and means for adjusting the value of the coefficient of friction. This adjusting means is arranged to prevent the variation calculation means from calculating the variation of the estimated vehicle speed, when the variation has exceeded a predetermined value more than a predetermined number of times in a predetermined period of time before the braking pressure control is initiated by the braking force control means, whereby the coefficient of friction can be estimated, even if all of the road wheels are locked simultaneously. However, according to the apparatus disclosed in the publication No.5-131912, if the braking force is gradually applied to the road wheel for example, the wheel speed will be also gradually decreased, so that it is difficult to detect the locking condition of the road wheel until the wheel speed is decreased to a relatively low speed. Therefore, the anti-skid control operation might be delayed thereby to cause all of the road wheels to be locked simultaneously. According to the conventional anti-skid control apparatus, the braking control system is provided for controlling the vehicle running on a road of a relatively high coefficient of friction (hereinafter, referred to as high-CF) when the anti-skid control is initiated. Therefore, if the gradual braking operation is made on a road of a relatively low coefficient of friction (hereinafter, referred to as low-CF), the wheel speeds of all the road wheels will be immediately decreased. Or, if the estimated vehicle speed is provided on the basis of the maximum wheel speed, for example, to perform the braking force control, the estimated vehicle speed will be rapidly decreased, with the wheel speed decreased, to be far from the actual vehicle speed.
Generally, when a running vehicle is braked, the axle loads which are applied to the front and rear portions of the vehicle respectively, will be different from each other due to the moving load caused by the braking operation. Therefore, the braking force applied to a front road wheel and-the braking force applied to a rear road wheel for locking all the road wheels simultaneously are not in direct proportion to each other, but in such a relationship as called an ideal braking force distribution, which varies depending upon the condition with or without load. If the braking force applied to the rear road wheel exceeds the braking force applied to the front road wheel, the directional stability of the vehicle will be deteriorated. In order to keep the braking force applied to the rear road wheel lower than that applied to the front road wheel and provide a distribution in close proximity to the ideal braking force distribution, a proportioning valve is provided between the rear wheel brake cylinder and the master cylinder, so that the braking force applied to the rear road wheel is generally set to be lower than the braking force applied to the front road wheel. Therefore, when the braking force is applied to the front and rear road wheels simultaneously, the front road wheel is likely to be locked in advance. This is irrelevant to the road surface conditions.
However, in the case where the rear road wheel is connected with the front road wheel through the engine as in the full time four-wheel drive vehicle, the wheel speed is caused to vary in a peculiar fashion, as explained below.
In general, the equation of motion of each road wheel on a road surface is as follows: EQU Im.times.Ar=CF.times.W.times.R-Tb
where "Im" is an inertia moment of a rotational system of a tire, "Tb" is a braking torque, "Ar" is an angular acceleration of a road wheel, "W" is a load of a tire, and "R" is a radius of the tire. "x" designates multiplication. In such a four-wheel drive vehicle as shown in FIG. 14, wherein its front road wheels and rear road wheels are operatively connected to an engine and a mission (shown as EG) through a center differential gear CD, a torsion is caused on a propeller shaft PS which transmits the driving force from the front road wheels to the rear road wheels. Therefore, the equation of motion which is applied to the case where a torque Td at the rear side (i.e., the torque of a rear differential gear RD) is transmitted to the center differential gear CD, i.e., a simulation model equation for the transmission of the torque as viewed from the driving side, is as follows: EQU Ip.times.np=Tp-Td, and EQU Td=Cp (np-nd)+Kp (Ap-Ad)
where "Ip" is an inertia moment of a rotational system including the propeller shaft PS, "Tp" is a torque of the propeller shaft PS, "np" is a rotational speed of the propeller shaft PS, "nd" is a rotational speed of the rear differential gear RD, "Cp" is a torsion damping coefficient, "Kp" is a torsional rigidity, "Ap" is an angle of rotation of the propeller shaft PS, and "Ad" is the angle of rotation of the rear differential gear RD. Thus, the torque is transmitted to the center differential gear CD, with a displacement of the propeller shaft caused by the torsion.
In the case where such a four-wheel drive vehicle as shown in FIG. 14 is running on a road surface of a relatively low coefficient of friction (low-CF), and where the coefficient of friction between the tire and the road surface decreases after it reached its peak value, the wheel acceleration is greatly decreased, because the inertia moment of the tire in its rotating direction is smaller than the inertia moment of the propeller shaft PS associated with the center differential gear CD. On the contrary, the inertia of the propeller shaft PS is relatively large. Therefore, a rotational speed differential (rotational angular differential) is caused between the rotational speed (np) of the propeller shaft PS and the rotational speed (nd) of the rear differential gear RD. In accordance with the rotational speed differential, the torque is transmitted to the propeller shaft PS so as to decrease the rotational speed of the propeller shaft PS. However, since the inertia of the propeller shaft PS is relatively large, the rotational speed of the propeller shaft PS is not decreased so much, whereas the torque is provided by its reaction to rotate the tire (in a direction for reducing the braking torque). Consequently, the wheel speed, which once tended to be decreased, tends to gain the speed again, thereby to cause a vibration or oscillation of the speed. In this connection, although the oscillation is caused when the vehicle is braked on the road surface of a relatively high coefficient of friction (high-CF), even if the braking torque is decreased by the reaction force, the tire torque is relatively large on that road surface. Therefore, the motion of the vehicle is not so much affected by the coefficient of friction of the road surface, until its position in the coefficient of friction vs slip rate characteristic comes to be located in such a region as being almost flat to reach the peak region. Accordingly, it is possible to distinguish the road conditions of the high-CF and the low-CF, on the basis of the oscillating state of the wheel speed differential between the rotational speed of the road wheel running on the high-CF road and that of the road wheel running on the low-CF road.
FIG. 15 illustrates the conditions during the gradual braking operation in the above-described four-wheel drive vehicle, and illustrates variations of a mean or average wheel speed of the front driven road wheels (hereinafter, referred to as front wheels) Vfa (hereinafter, referred to as a front mean speed Vfa), and a mean wheel speed of the rear driven road wheels (rear wheels) Vra (a rear mean speed Vra), in the case where the vehicle is running on the road surface of the low-CF as shown in the upper section of FIG. 15, and where the vehicle is running on the road surface of the high-CF as shown in the lower section of FIG. 15. On the low-CF road, when all of the road wheels are braked, the front mean speed Vfa is decreased in advance as shown by a phantom line. Then, the rear mean speed Vra is decreased as shown by a solid line, and at the same time the front mean speed Vfa is once increased. Thereafter, the front and rear mean speeds Vfa, Vra are decreased varying respectively to generate an oscillation of the speed differential between the front and rear mean speeds Vfa and Vra. On the high-CF road surface, however, as shown in the lower section in FIG. 15, the front and rear mean speeds Vfa, Vra are decreased in substantially the same fashion. Comparing the upper section with the lower section in FIG. 15, therefore, if it is possible to determine the characteristic in which the front and rear mean speeds Vfa, Vra are decreased varying respectively, the road surface could be estimated to be of the low-CF, at least.